I see a lot of black transoms out there. I think there is a tendency to overprop and get hull speed at lower than full throttle / full RPM. Especially on sailboats. Power boaters tend to spend a lot mor time on this. IMHO ...thats all

What I have been told is that it blows out any accumulating carbon in the exhaust hose. Someone told me that this is especially important if there's a u-bend in the exhaust hose because carbon can particularly build up there.

That doesn't from from my mechanic. It comes from a friend who has the same engine as mine, which is a 3 piston, 20 hp fresh water cooled Yanmar.

Not sure why anyone would think all diesels are operated in the same manner. They have been around for many decades in sizes ranging from a lunchbox to a locomotive.

It's a mistake to ask the op's question in a generic sense and not in regard to a specific engine.

What engine is the OP wishing to know about?

The service manual for mine states to motor away at 50% rated rpm (1800 rpm) for the first 5 minutes (after 3 minutes warmup at 1200 rpm), and then cruise at 80% of rated rpm (~2900 rpm), and when needed use full rated rpm (3600 rpm) for no more hour at a time. So it's actually OK to redline the engine for an hour at a time. And cruise at 80% of redline all day long.

Further, it states after docking to idle for 5 minutes to cool the engine and then throttle up to max for 30 seconds and back to idle then shut down.

"When the engine is operated at a low idling speed (below 1000rpm) for a long time (over 2 hours), excess carbon and fuel residue tends to accumulate due to incomplete combustion.

Carbon deposits on the injection holes of the fuel injection valve, exhaust valave, the turbine blades of the turbo-charger etc. cause a drop in engine output, knocking and other troubles. To prevent these problems, be sure to blow off the carbon accumulations by full speed operation.

Operate the engine at over 2500rpm for one minute in every two (2) hours of continuous low idling operation."

9 years ago I rebuild my Perkins 4-108 with a mechanic friend. Everything new! While bleeding it, friend was in the engine compartment and me cranking at the helm at WOT. As usual, with the pain in the ass to bleed Perkins, it took several tries. At one point I decided to give the starter a good cool off period and walked away leaving behind the lever at WOT.
The mechanic stayed behind to bleed once more. Before I returned he jumped the solenoid and she fired up racing away at full gas. Before he could get topsides to ease off on the gas, several seconds passed with the engine running at full revs.
I was shocked when he told me what happened but I have never, ever had any issues with this engine so far.
And whatever Cabo-Dude said in post # 4

I tried to explain to an owner that excessive running under no load situations is not good for the engine......on one occasion I told this person that running out of gear at WOT will cause smoking due to overfueling.......The vessel had been taken for a seatrial.....no smoke under load WOT.......

Person got another mechanic who said the compression is a "couple hundred low on all cylinders"....engine is now torn apart......

I've head some people say you need to run a diesel hard, otherwise you may shorten it's life.

I've heard some people say, they only cruise at a very moderate rpm, and that constantly running at a high rpm is bad for it.

And I've recently heard someone say that the first one is only true with old diesels.

So which one is actually true?

When it comes to a typical sailboat diesel, terms like hard and high rpm are very relative.

My Universal is 1 litre, and produces 24hp at 3K rpm.

Contrast that to a typical automotive gasoline engine, producing maybe 100hp per litre at 6.5K rpm with a turbocharger.

Mechanical wear is obviously going to be less of a worry, and take many more hours of running, with the marine diesel. Here the problem is more likely to be carbon and soot buildup from the engine not working hard enough.

I've heard its bad to run them at a constant rev rate for long periods... either raise or drop revs by a few hundred for a while...

I think that comment applies to running in any engine. Avoid long periods at constant, high rpm and load. The best initial running-in involves driving gently mostly but progressively extending the rpm for short periods. That way you loosen up any parts that are tight without causing excessive heat in the tight spots.

I did this for the first 500 miles on my VW Jetta's 2.5 and the engine is great now - loads of power and zero oilconsumption. Maybe it would have been like this anyway? Who knows?

this discussion has repeated on here many times. There seems to be no one answer. Truckers let their engines idle for as much as 1/3 of the life of the engine, and nothing is more of a concern to them than engine life. Many identical marine diesels are rate at differnt max rpm's based on the stated use. (recreational, commercial and continuous duty) The same engine for continuos duty may be rated at 2600 rpm and for rec use 3500 rpm. That would indicate to me that lower rpm is better.

__________________"I spent most of my money on Booze, Broads and Boats. The rest I wasted" - Elmore Leonard

Currently have about 10,000 hours each, and 95% of that time has been at a power level of 10-20%, based on fuel flow rate. No special effort to rev to max occasionally. Lots of "trawler miles" at 6 kts.

I have to ask

If 95% of your time is at 10-20% load to produce 6 knots, why not have smaller, lighter, cheaper engines that push it at 6 knots?

That was my reasoning behind having 65hpx2 (peak torque 50hp at 1600rpm) in a 50ft cat, because most of the time you are running at slow speed anyway.

I think by far the best advice is to read the manual for YOUR engine. They vary a lot in design, components, ratings, etc. What's right for one may be wrong for another.

As an example, some of the "advice" given here to always run at 85% to 95% load directly contradicts what the manual for my engines says. I'm sure that "advice" is correct for some engine out there in the world, but it would be very bad advice for mine and would void the warranty.